scroll.js

var map = new ol.Map({
    target: 'map',
    layers: [
        new ol.layer.Tile({
            source: new ol.source.OSM()
        })
    ],
    view: new ol.View({
        center: ol.proj.transform([-0.92, 52.96], 'EPSG:4326', 'EPSG:3857'),
        zoom: 6
    })
});

var popup = new Popup();
map.addOverlay(popup);

map.on('singleclick', function(evt) {
    var content = '<p>If the popup content is quite long then by default it will scroll vertically.</p>';
    content += '<p>This behaviour together with the minimum width and maximum height can be changed by overriding the rules for the CSS class <code>.ol-popup-content</code> defined in <code>src/ol-popup.css</code>.</p>';
    content += '<hr />';
    content += '<p><em>This text is here to demonstrate the content scrolling due to there being too much content to display :-)</em></p>';
    popup.show(evt.coordinate, content);
});